help about how to use latency options

help about how to use latency options

[Larry Chen]

Hi list,

I referred to the docs and examples but I'm still hard to understand how 
to use latency_target, latency_window and latency_percentile.
Could any one help explain it in detail?
Thanks in advance

Sorry for that this letter might be resent to the list because my
misoperation of subscribing fio mail list.

Regards,
Larry

Re: help about how to use latency options

[Sitsofe Wheeler]

Hi,

On 11 April 2018 at 07:48, Larry Chen <lchen@suse.com> wrote:
>
> I referred to the docs and examples but I'm still hard to understand how to
> use latency_target, latency_window and latency_percentile.
> Could any one help explain it in detail?
> Thanks in advance
>
> Sorry for that this letter might be resent to the list because my
> misoperation of subscribing fio mail list.

These options are used to probe the highest iodepth usable such that
the I/Os still have a latency equal to or below that of latency_target
on the basis that more asynchronous I/Os being sent at the same time
the higher each I/Os latency will be (this is what
http://fio.readthedocs.io/en/latest/fio_man.html#cmdoption-arg-latency-window
is trying to say). latency_window controls the time window that I/Os
are considered over. latency_percentile is used to adjust the
sensitivity to outlier I/Os during this process so a single moment
high spike is ignored.

Any clearer?

-- 
Sitsofe | http://sucs.org/~sits/

Re: help about how to use latency options

[Larry Chen]

Hi 何蒙,

Thanks for your reply, your explain on latency_window is excellent.

The docs says it "will attempt to find the max performance point",
but it doesn't say any attempt to make some changed to fit in the 
latency_target.

Thanks
Larry


On 04/11/2018 03:28 PM, 何蒙 wrote:
> Hi Larry
> It is my understanding that:
> If latency_target=100ms, latency_target=90, fio will attempt to make 
> 90% of all IOs lower than latency_target，
> If lantency_window=1s，fio will calculate all IOs in the 1st second，if 
> don't meet the target (90% latency less than 100ms)
> fio will do some changes to meet the target in next seconde.
>
> 2018-04-11 14:48 GMT+08:00 Larry Chen <lchen@suse.com 
> <mailto:lchen@suse.com>>:
>
>     Hi list,
>
>     I referred to the docs and examples but I'm still hard to
>     understand how to use latency_target, latency_window and
>     latency_percentile.
>     Could any one help explain it in detail?
>     Thanks in advance
>
>     Sorry for that this letter might be resent to the list because my
>     misoperation of subscribing fio mail list.
>
>     Regards,
>     Larry
>     --
>     To unsubscribe from this list: send the line "unsubscribe fio" in
>     the body of a message to majordomo@vger.kernel.org
>     <mailto:majordomo@vger.kernel.org>
>     More majordomo info at http://vger.kernel.org/majordomo-info.html
>     <http://vger.kernel.org/majordomo-info.html>
>
>

Re: help about how to use latency options

[Larry Chen]

Hi Sitsofe,
Thanks for your explain.
I found that if latency_target is specified, latency_window must be 
along with latency_target option.
If not, fio will exit immediately.
Why the case is like that?

Thanks
Larry

On 04/11/2018 03:59 PM, Sitsofe Wheeler wrote:
> Hi,
>
> On 11 April 2018 at 07:48, Larry Chen <lchen@suse.com> wrote:
>> I referred to the docs and examples but I'm still hard to understand how to
>> use latency_target, latency_window and latency_percentile.
>> Could any one help explain it in detail?
>> Thanks in advance
>>
>> Sorry for that this letter might be resent to the list because my
>> misoperation of subscribing fio mail list.
> These options are used to probe the highest iodepth usable such that
> the I/Os still have a latency equal to or below that of latency_target
> on the basis that more asynchronous I/Os being sent at the same time
> the higher each I/Os latency will be (this is what
> http://fio.readthedocs.io/en/latest/fio_man.html#cmdoption-arg-latency-window
> is trying to say). latency_window controls the time window that I/Os
> are considered over. latency_percentile is used to adjust the
> sensitivity to outlier I/Os during this process so a single moment
> high spike is ignored.
>
> Any clearer?
>

Re: help about how to use latency options

[Sitsofe Wheeler]

Hi,

fio needs to know what the period it should be sampling the latency
over. If the latency was OK in the current latency window it can up
the iodepth by one and start a new latency window thus repeating the
process. Without a window you will (by definition) have no samples to
look at!

On 11 April 2018 at 10:01, Larry Chen <lchen@suse.com> wrote:
> Hi Sitsofe,
> Thanks for your explain.
> I found that if latency_target is specified, latency_window must be along
> with latency_target option.
> If not, fio will exit immediately.
> Why the case is like that?

-- 
Sitsofe | http://sucs.org/~sits/

Re: help about how to use latency options

[Larry Chen]

Thanks
  :-)

On 04/16/2018 04:28 PM, Sitsofe Wheeler wrote:
> Hi,
>
> fio needs to know what the period it should be sampling the latency
> over. If the latency was OK in the current latency window it can up
> the iodepth by one and start a new latency window thus repeating the
> process. Without a window you will (by definition) have no samples to
> look at!
>
> On 11 April 2018 at 10:01, Larry Chen <lchen@suse.com> wrote:
>> Hi Sitsofe,
>> Thanks for your explain.
>> I found that if latency_target is specified, latency_window must be along
>> with latency_target option.
>> If not, fio will exit immediately.
>> Why the case is like that?